1. Field of the Invention
The present invention relates generally to optical switches for telecommunications and related applications, and particularly to multi-stage optical cross-connects.
2. Technical Background
A cross-connect is generally a big, complex switch, or, more specifically, a device for connecting any one of N signal inputs to any one of M signal outputs. Large cross-connects in practical use today employ complex electronic switching. But because optical signals can carry data for much greater distances at much greater rates than electronic signals, development of optical cross-connects is proceeding in response to ever increasing demand for higher capacity, more efficient telecommunications.
The basic logical structure of a cross-connect is shown schematically in FIG. 1. Any one of the input lines 20 may be selectively connected to any one of the output lines 30. This may be achieved, for example, by a plurality of switching devices positioned at respective cross-points 40 between the input and output lines, where each switching device allows a connection to be selectively established between the associated input and output lines.
In an optical cross-connect, the input lines 20 of FIG. 1 may take the form of waveguides of various forms including those formed on or in a substrate, or optical signal beam paths for unconfined beams propagating in free space or in another medium, or the like. Optical signals entering such a cross-connect along the input lines 20 in the direction A, for example, may be redirected at selected ones of the cross-points 40 so as to exit the switch along desired ones of the output lines 30 in direction B, for example.
Depending on the particular switching devices employed and other factors, the geometry of a typical optical cross-connect may vary from that shown in FIG. 1. For example, the intersection of waveguides or beam paths may be at angles other than right angles, and inputs may arrive in more than one direction A, and outputs may leave in more than one direction B, as shown in FIG. 2, for example. Inputs and outputs may also be interchangeable and/or physically indistinguishable in an optical switch, so one line may function as either or both. Accordingly, while xe2x80x9cinputxe2x80x9d and xe2x80x9coutputxe2x80x9d will be used for convenient reference herein, it is to be understood that these are equivalent terms in the context of the optical switches as described and claimed herein, and are understood not to limit the structure and functionality of the inventive devices thus described.
Devices employing frustrated total internal reflection are also known to those of ordinary skill in the art. All these and such other switching devices as known to those of skill in the art may be used in optical switches of the type shown schematically in FIGS. 1 and 2.
One example of such switching devices are small mechanical mirrors known as xe2x80x9cMEMSxe2x80x9d (Micro-Electro-Mechanical Systems) mirrors. MEMS mirrors can be used with either a beam-based or a wave-guide based switch. In either case, the mirrors are positioned at or near the cross-points 40 and are controllably moveable so as to be selectively inserted at the respective cross-points. With a mirror inserted at a selected cross-point, a beam or a guided wave arriving at that cross-point is redirected from direction A to direction B at that cross-point. In a switch employing inputs from two directions A as in FIG. 2, double-surfaced mirrors can be used. An example wave-guide and MEMS-mirror switch is disclosed in U.S. patent application Ser. No. 99/24,591, filed Oct. 20, 1999, assigned in common with the present application and incorporated herein by reference. An example free-space beam and MEMS mirror optical switch is disclosed in U.S. Pat. No. 5,960,132 also incorporated herein by reference.
Another example switching device suggested for use in optical switches of the type shown schematically in FIG. 1 is a fluid injection device. By injecting a fluid into (or removing a fluid from) a cross-point of a waveguide structure, the index of refraction can be varied at the cross-point so as to reflect an incoming guided wave arriving at the crosspoint into the associated output waveguide. The construction and use of such a switching device is disclosed, for example, in U.S. Pat. No. 5,978,527 incorporated herein by reference.
Another example switching device suggested for use in optical switches of the type shown schematically in FIG. 1 uses liquid crystals formed within the waveguides at the respective cross-points. The construction and use of such devices is disclosed in U.S. patent application Ser. No. 09/604,039 and U.S. patent application Ser. No. 09/431,430, commonly assigned with the present application and incorporated herein by reference.
Devices employing techniques such as photonic crystals or frustrated total internal reflection are also known to those of ordinary skill in the art. All these and such other switching devices as known to those of skill in the art may be used in optical switches of the type shown schematically in FIGS. 1 and 2.
Where waveguides are used (rather than beams) for light propagation within the switch, the crossing pattern of the input and output lines shown in FIGS. 1 and 2 may be folded back on itself, such as is as shown schematically in FIG. 3. An example of a switch of this design using MEMS mirrors is disclosed in U.S. Pat. No. 5,148,506, incorporated herein by reference.
While all of the above-described switching devices may be useful in switch designs such as those of FIGS. 1-3, as the scale of the switch increases, these switch designs are less practical. The number of cross-points typically increases as the number of inputs times the number of outputs, making large-scale switches, such as 1024xc3x971024 for example, difficult to implement.
It is well known that larger cross-connects can be formed from several smaller cross-connects linked together. One way of linking non-blocking cross-connects together to produce a larger non-blocking cross-connect is illustrated in FIG. 4.
FIG. 4 shows an example layout for an arrangement known as a three-stage Clos network. The network shown, as a whole, provides an Nxc3x97N non-blocking cross-connect. Stage 1 includes multiple cross-connects, each with n inputs and 2nxe2x88x921 outputs, as illustrated for cross-connect number 1 of Stage 1. The N inputs to the network are received by a total of N/n of these cross-connects in Stage 1. For each of the N/n cross-connects of Stage 1, each of the 2nxe2x88x921 outputs are routed to a respective Stage 2 cross-connect, as illustrated for cross-connect 2 of stage 1. Stage 2 includes 2nxe2x88x921 cross-connects, one for each output of the Stage 1 cross-connects. Each cross-connect of stage 2 has N/n inputs and outputs. For each of the 2nxe2x88x921 cross-connects of stage 2, each of the N/n outputs are routed to a respective Stage 3 cross-connect, as illustrated for cross-connect 2 of Stage 2. Stage 3 includes N/n cross-connects, one for each output of the Stage 2 cross-connects. Each of Stage 3 cross-connects has 2nxe2x88x921 inputs and n outputs. The n outputs of all of the Stage 3 cross-connects together provide the N outputs of the network. As may be appreciated from FIG. 4, the interconnections between the stages in such a multi-stage switch can be complex.
The present invention includes, in one aspect, a multi-stage optical cross-connect including a first stage having a plurality of first stage sub-switches each structured and positioned so as to receive inputs along a plurality of first stage input directions and to send outputs along a plurality of first stage output directions, and a second stage including a plurality of second stage sub-switches each structured and positioned so as to receive inputs along a plurality of second stage input directions and to send outputs along a plurality of second stage output directions, and a third stage including a plurality of third-stage sub-switches each structured and positioned so as to receive inputs along a plurality of third stage input directions and to send outputs along a plurality of third stage output directions, wherein, for each first stage sub-switch, each of the output directions of said sub-switch is each aligned with an input direction of a respective second stage sub-switch, and each of the output directions of said sub-switch is each aligned with an input direction of a respective third stage sub-switch. When each sub-switch is configured as a simple Mxc3x97N non-blocking cross-connect of the type shown in FIG. 1, this results in a greatly simplified three-stage optical switch with at most three changes of direction and three switch devices within each optical path. This cross-connect may be implemented all in a single plane or in multiple planes or on multiple substrates according to the various embodiments disclosed herein.
In another aspect, the invention includes a first, second, and third stages each including multiple layers, each layer including a non-blocking cross-connect sub-switch having inputs and outputs. The first-stage layers are arranged such that the inputs of the first-stage layers are arranged in a two-dimensional first-stage input matrix and such that the outputs of the first-stage layers are arranged in a two-dimensional first-stage output matrix. The second- and third-stage layers are arranged similarly, thus providing second and third stage input and output matrices. Each of the outputs of the first-stage output matrix is optically connected to a respective input of the inputs of the second-stage input matrix via respective non-interleaving first-to-second-stage optical connections, and each of the outputs of the second-stage output matrix is optically connected to a respective input of said inputs of the third-stage input matrix via respective non-interleaving second-to-third-stage optical connections. This provides a greatly simplified interconnection scheme for a three-stage optical cross-connect. The optical connections may take various forms including xe2x80x9cpigtailedxe2x80x9d waveguides, or direct coupling of planar waveguides, or free-space beam propagation.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.